WHETHER or not you believe in "climate change", the facts indicate our climate is changing.

A record 45.7C in Sydney, 41.8C in Hobart. The longest stretch of hot days nationwide. Devastating fires in Tasmania, Victoria and NSW. Tornadoes and record rain in parts of Queensland.

Extreme weather is part of the Australian reality; it is why Dorothea Mackellar's poetic words about our land - "her beauty and her terror" - resonate still.

But the big question is whether these extremes are becoming more frequent and more terrible as part of climate change driven by a warming planet.

President Barack Obama believes so, and he has put climate change back on the international agenda, declaring the US must lead the race for new technology to reduce carbon emissions.

"We will respond to the threat of climate change, knowing that the failure to do so would betray our children and future generations," Mr Obama said in his inaugural address.

"Some may still deny the overwhelming judgment of science, but none can avoid the devastating impact of raging fires, and crippling drought, and more powerful storms."

He could have been talking about Australia. But is it really climate change, or just the normal variability of summer and weather conditions, that we cannot recognise because of inadequate records?

Climate scientists now focus on the consequences, rather than the simple facts, of warming playing out every day. In Australia, the Climate Commission has responded to calls for clarity with a concise report: Off the charts: Extreme Australian summer heat.

Author Professor David Karoly said the length, extent and severity of the January heatwave was "unprecedented in the measurement record".

"While hot weather has always been common in Australia, it has become more common and severe over the past few decades," he said. "There has been a significant increase in the frequency of hot days - days over 35C - and hot nights over the last 50 years.

"The frequency of record hot days has been more than three times the frequency of record cold days in the past 10 years."

Australia's average temperature has risen 0.9C since 1910, the Bureau of Meteorology says.

Climate monitoring manager Dr Karl Braganza, at the Bureau's National Climate Centre, said extreme heat records were "breaking with increasing frequency, particularly in the last two decades" as a result.

"We don't really view one event in isolation - when we're looking at climate change, we're looking at how many of these events we're getting," he said.

"Climate change doesn't cause one event but it will influence how often you see that type of event. There is this really clear trend of increasing frequency of extreme heat. That's salient for future climate change."

Globally, the 10 hottest years on record have all occurred in the past 15 years, NASA says.

The scientists are convinced that climate change is making extreme hot days, heatwaves and bushfire weather worse.

Mr Obama made the link between warming and extreme weather, persuaded by the latest data from the National Climatic Data Centre at the US National Oceanic and Atmospheric Administration. Earlier this month, NOAA revealed that last year was the hottest on record for the US.

It was also the second-most extreme. Scientists calculated the average temperature for the whole country and rated the year on the US Climate Extremes Index, which provides a measure of extremes in temperature and rainfall, as well as landfalling tropical cyclones.

The year 2012 was nearly twice the average value on the extremes index, second only to 1998.

The United Nations Intergovernmental Panel on Climate Change addressed the issue of extreme events and disasters in a special report, which was released in full last March. The report assessed science that was not available at the time of the last major update for policy-makers, the Fourth Assessment Report, released in 2007.

At that time, former Vice-President Al Gore was promoting his documentary, An Inconvenient Truth, and Mr Obama was running his first election campaign. Now the world's climate scientists have observed a rise in the number - and duration - of warm spells or heatwaves in many parts of the world.

Longer and more intense droughts are to be expected but when it does rain, it pours and floods become a bigger problem.

The flow-on effects are numerous. Insurance companies which have been paying out massively on a string of national disasters will face pressure for higher premiums, power costs will rise as "peak demand" grows due to the growth in air-conditioning, and the toll on human life from hotter summer days may rise.

Forecasters at Melbourne's RMIT University say that by 2050 there will be 6200 extra heatwave deaths for Melbourne and 1926 additional deaths in Adelaide, above the predicted number if no climate change took place.

UN Secretary-General Ban Ki-moon will press world leaders to agree on a binding deal against climate warming by 2015.

Last week he told the UN General Assembly that mobilising action on climate change is now a "priority". "Next year I intend to invite leaders of the world ... to mobilise the necessary political determination to adopt by 2015 a strong and binding instrument on climate change," he said.

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